In a duplex wireless communication system, a radio network node and a wireless communication device communicate with one another in both the uplink direction (from the device to the radio node) and the downlink direction (from the radio node to the device). The duplexing system is referred to as a frequency-division duplexing (FDD) system if the radio node and the device communicate in the uplink direction at the same time as communicating in the downlink direction, but over different carrier frequencies or bandwidths. By contrast, the duplexing system is referred to as a time-division duplexing (TDD) system if the radio node and the device communicate in the uplink direction over the same carrier frequency or bandwidth as communicating in the downlink direction, but at different times.
More specifically, a TDD system employs different radio resources in the time domain for communication. In many TDD systems, these time-domain radio resources are referred to as subframes, with a defined number of consecutive subframes (e.g., 10) being referred to as a frame. See, for instance, FIG. 1, which shows the subframes of a TDD system based on the Long Term Evolution (LTE) standard. Regardless, some subframes within any given frame are allocated for uplink (UL) communication and other subframes within the frame are allocated for downlink (DL) communication (with a switch between downlink and uplink occurring in a special subframe). This description uses the term “subframe” interchangeably with time-domain radio resources, and therefore should be understood as applicable to any TDD system, not just those based on LTE.
A dynamic TDD system employs different TDD configurations (also referred to as uplink-downlink configurations). Different TDD configurations define a different relative number and/or arrangement of UL and DL subframes within a frame. See, for instance, the 7 different TDD configurations shown in FIG. 2 for LTE-TDD systems. A configuration is asymmetric if it has more UL subframes than DL subframes so as to be UL-heavy, or vice versa. An UL-heavy configuration has greater capacity for UL traffic than DL traffic, while a DL-heavy configuration has greater capacity for DL traffic than UL traffic.
Notably, any given radio network node of a dynamic TDD system dynamically adapts its TDD configuration. This adaptation may occur on a relatively quick basis (e.g., on a frame by frame basis) to accommodate the node's instantaneous amount of DL traffic and UL traffic, and/or on a relatively slower basis in order to accommodate the node's UL/DL traffic pattern or characteristics. In at least some systems, a radio network node dynamically selects its TDD configuration from a set of predefined configurations. This differs from FDD systems in which a given bandwidth is allocated to either UL or DL regardless of the traffic and need of a node. Regardless, each node performs this dynamic TDD configuration adaptation independently.
The cost of dynamic TDD configuration (i.e., UL/DL dynamic resource adaptation) is the cross interference between UL and DL that arises when neighboring nodes (i.e., base stations) use different TDD configurations. This cross interference is base station to base station interference and/or device to device interference, and does not occur in FDD systems. In some cases, these interferences can become very severe and detrimentally impact the system performance.
Known approaches to TDD system implementation have not contemplated TDD systems being used in conjunction with so-called dual or multiple connectivity. Envisioned thus far outside the context of any TDD system, a multiple-connected wireless communication device connects to multiple different radio network nodes. The device transmits to one or more of the radio nodes while simultaneously receiving from one or more of the other radio nodes. In essence, multiple independent communication links (e.g., up to at least the radio link control, RLC, layer) are associated with the different radio network nodes as different transmission/reception points. In practice, the network points may be in different layers of the system, as shown for instance in FIG. 3, where a device is dually connected to both a high power base station and a low power base station. However, inter-pico dual connectivity may be practiced as well. Thus, multiple connectivity can be used in many different scenarios and in different ways (e.g. the device may be simultaneously connected to multiple network points on the same or separate frequency, the UL and DL transmissions may be decoupled or not, etc).